1. Field of Invention
This invention relates to the field of passive radio frequency (RF) signal emitter ranging using a plurality of emitter signal detecting platforms extracting and exchanging information about RF signal features from said RF signal emitter.
2. Description of the Related Art
Robust radio emitter passive ranging requires a precise measurement of the time of arrival of features of an RF emitter signal to two or more detecting platforms. A high signal to noise ratio at (at least) two detecting platforms for the features of the emitter signal are generally required for a robust estimation of (ambiguous) range. Historically, the signal waveform of an emitter is estimated by assuming cyclostationary features. These features are derived from time consuming examination of assumed time domain features in likely portions of the electromagnetic spectrum. The time domain features of the signal to be examined include various parameters such as bandwidth of the signal, its location within the operating spectrum, peak pulse shape, rise times or some other defining, time marking artifact. When examined in the frequency domain, these features transform into markers that can be used to identify the signal.
The signal may be present in a unknown form, within a narrow portion of the spectrum and present a relatively low signal to noise ratio at the receiving platform. Thus, detection implies a time consuming process of searching for signal features using narrow detection bandwidths. With marginal signal to noise ratio, uncertainty mounts as to the detection of the signal and as to its features. Consequently, it is desired to minimize the time for robust detection of an emitter presenting low signal to noise ratio to a plurality of platforms cooperatively interconnected for the detection of said emitter.
A system and method for locating the position of an emitter is described. The emitter emits a cyclostationary signal having time domain features and noise, in an operating band within a first frequency range. The system comprises a first platform having a first motion. The first platform is located at a first range from the emitter, and a second platform having a second motion, is located at a second range from the same emitter.
Typical platforms, part of the system, such as the first and second platform, have tunable bandpass means, such as a bandpass filter, or FFT channelizer, for detecting the cyclostationary signal within the operating band. The tunable bandpass means extracts the time domain features and noise from the cyclostationary signal to obtain a result.
The tunable bandpass means for detecting time domain features of the cyclostationary signal, for example, comprises an antenna coupled to a downconverter having an analog output; the analog output is presented to an analog to digital converter for digitizing said analog output to a digital format. The digital format is presented to an FFT channelizer. The FFT channelizer implements a plurality of bandpass filters. The channelizer generates a result, a sequence of digital words representative of the output from the bandpass filters.
The result from the channelizer is thresholded to reduce noise thus generating a second result. The second result comprises digital words. The thresholding function compares each of the digital words generated by the channelizer to a digital threshold, for example, the lowest significant 2 bits. The threshold is computed adaptively. If a particular digital word is less than the threshold, then the digital word is set to zero. Conversely, when the digital word is more than the digital threshold, it is left unchanged.
An FFT is used for transforming the thresholded second result to the frequency domain. One or more templates indicative of known emitter features are correlated with the output of the FFT. This correlation generates a series of frequency domain markers showing where in the frequency domain an overlap exists between the template and the actual received signal.
The frequency domain markers are used for (ambiguously) computing the first range.
A transmitter is used for transmitting the frequency domain markers to the second platform using a wireless link. The second platform receives the frequency domain markers from the first platform. The wireless link operates within a second frequency range. This second frequency range is separate and distinct from the first frequency range.
The frequency domain markers detected by the first platform are used to enhance detection of the emitter, its cyclostationary signal, and the time domain features and noise at the second platform.
The second platform has the means for extracting frequency domain markers from the emitter""s cyclostationary signal, and is identical to to the means used by the first platform. As with the first platform, the second platform has means for computing its (second)(ambiguous) range to the emitter. Like the first platform, it transmits its results, such as the second range, using the wireless link, to all members of the system sharing the wireless link, such as the first platform.
The motion, that is position, velocity and acceleration of the first platform is computed using a Kalman filter updated from motion data supplied from accelerometers located on the first platform at time intervals. The length of the time intervals is determined by the level of accuracy desired. Every 1 msec is typical.
Similarly, the second motion of said second platform is computed using a second Kalman filter on board the second platform, and is updated from motion data supplied from second accelerometers located on the second platform at time intervals.
Another aspect of the system is the computation of a confidence factor indicative of the ratio of the noise to the time domain features of said cyclostationary signal presented to a platform by an emitter. This confidence factor transmitted to other platforms using the wireless link. This signal to noise calculation gives an idea of the quality of the detection using a particular template.
The information carried by the wireless link is used to enhance the detection of a signal at other platforms. Upon receiving frequency domain markers detected at the first platform and transmitted to the second platform, the second platform adjusts its templates in response to the received frequency domain markers, and in response to its second motion and changes in its range with respect to the emitter. This reduces the need to try various templates corresponding to time domain features at the second platform.